1. Field of the Invention
The present invention relates in general to a control apparatus for a vehicular drive system, and more particularly to shifting control of the vehicular drive system by the control apparatus according to a selected one of two different operating states of the vehicular drive system.
2. Discussion of Prior Art
There is known a control apparatus for a vehicular drive system including an electrically controlled differential portion, a transmission portion, and a differential-state switching device configured to switch a differential mechanism of the electrically controlled differential portion between a differential-state in which the differential portion is operable to perform a differential function, and a non-differential state in which the differential portion is not operable to perform the differential function. JP-2006-9942 A discloses an example of such a vehicular drive system control apparatus. In a vehicular drive system as disclosed in the above-identified publication, the differential mechanism is constituted by a planetary gear set, for example, and a major portion of a drive force of an engine is mechanically transmitted to drive wheels of a vehicle owing to the differential function of the planetary gear set, while the remaining portion of the drive force of the engine is transmitted to a first electric motor from which an electric energy is transmitted to a second electric motor through an electric path therebetween, so that the planetary gear set functions as the electrically controlled differential portion in which the differential state between the rotating speeds of the input and output shafts is electrically variable. The vehicular drive system is controlled by the control apparatus to drive the vehicle with the engine held in an optimum operating state to maximize the fuel economy of the vehicle. The vehicular drive system disclosed in the publication is provided with a step-variable automatic transmission disposed between the output shaft of the differential mechanism and the drive wheels, for the purpose of minimizing the required size of the second electric motor.
A step-variable transmission is provided with a plurality of coupling devices which are selectively engaged and released to perform a shifting action of the step-variable transmission. In the presence of variations of the step-variable transmission with respect to nominal specification, the step-variable transmission has a risk of generation of a shifting shock if the shifting action is performed at a nominal point of shifting. To prevent or reduce this risk of shifting shock, a learning control of the shifting action of the step-variable transmission is widely implemented. For instance, the learning control involves a control of engaging pressures of the coupling devices and/or a control of timings of engaging and releasing actions of the coupling devices in a transient state of the shifting action of the step-variable transmission, depending upon a specific running condition of the vehicle such as an input torque of the step-variable transmission.
Such a learning control for reducing the shifting shock is preferably implemented for the transmission portion of the vehicular drive system including the above-described electrically controlled differential portion as well as the transmission portion. As described above, the electrically controlled differential portion has the two operating states, that is, the differential state and the non-differential state, so that the vehicular drive system as a whole has two operating states corresponding to the two operating states of the differential portion. If the learning control of the shifting action of the transmission portion is implemented in a predetermined manner irrespective of the presently selected one of the differential and non-differential states of the differential portion, that is, without taking account of the presently established differential or non-differential state of the differential portion, the learning control does not necessarily prevent or reduce the risk of shifting shock of the transmission portion, due to a difference between the differential and non-differential states of the electrically controlled differential portion.
If the learning control is implemented in different manners suitable for the respective differential operating states of the vehicular drive system as a whole, the learning control provides a better result in the prevention or reduction of the shifting shock, than the learning control implemented in the predetermined single manner irrespective of the operating states of the vehicular drive system. In this case, however, the manner of the learning control frequently changes upon a change of the operating state from one state to the other, so that the time allowed for each opportunity of the learning control tends to be short, giving rise to a requirement for a long length of time for a satisfactory or successful result of the learning control, whereby the transmission portion may suffer from a shifting shock until the satisfactory result of the learning control is obtained.